As a velocity controller for induction motors, constant-V/F-control-system inverters that control the ratio of output voltage V to output frequency F to be constant have been widely utilized. With regard to these inverters, when they are restarted, with electric power being restored, after they have been stopped due to the occurrence of instantaneous power failure, or when they are initiated under the circumstances that an induction motor is coasting on external force while the inverter is stopped, it is necessary to re-accelerate the coasting induction motor in such a manner that the rotating frequency F of the induction motor approximately coincides with the output frequency f of the inverter. Accordingly, when the constant-V/F-control-system inverters are initiated, it is necessary to learn the output frequency f of the inverter and the rotating frequency F of the coasting induction motor; therefore, the rotating frequency F has been obtained by providing a velocity detector such as a tacho-generator, or by detecting residual voltage in the induction motor and carrying out calculation base on its frequency components.
Moreover, Japanese Laid-Open Patent Publication 1991-3694 discloses induction motor controllers that realize the object of obtaining the rotating frequency F of a coasting induction motor, without requiring special induction motors equipped with a velocity detector such as a tacho-generator, or dedicated voltage detectors such as a voltage detecting transformer for detecting residual voltage of an induction motor.
Japanese Laid-Open Patent Publication 1991-3694 discloses: when, with an induction motor being coasting, a dc current commanding signal is outputted from a current commanding section, output current from an inverter section is controlled by a control signal system, based on difference between the dc current commanding signal and a detected signal that is a detected value of the output current from the inverter section; in this situation, ripple components are produced in the control signal system; and, by means of extracting the ripple components, the rotating status of the coasting induction motor is learned.
In addition, Japanese Laid-Open Patent Publication 1991-3694 discloses (12th line from the first line in the bottom-right paragraph of page 7): reversing the output-current polarity of an inverter section by reversing the phase angle θ of the current command from 0 degree to 180 degrees in 10 ms after the detection of the rotating status of a coasting induction motor has started, the amplitudes of ripple components that superpose on a voltage command are significantly increased.
What Japanese Laid-Open Patent Publication 1991-3694 discloses utilizes the fact that, because the control signal system undergoes a disturbance accompanied by steep current changes, by reversing the polarity of current inputted to the coasting induction motor, in process of supplying the current, the amplitudes of the superposing ripple components increase. There is inconsistency in ripple amplification utilized for velocity detection, and, in particular, when the frequency is low, the ripple is significantly small; therefore, there has been a problem in that the rotating frequency F of the coasting induction motor cannot be detected with a high degree of accuracy.
The present invention solves the problem discussed above, and its prime object is to detect with a high degree of accuracy the rotating frequency F of the coasting induction motor, regardless of unevenness in the ripple amplification utilized for velocity detection.